Generally when we catch visual information including objects or letters from a transparent substrate or from the transparent substrate of a mirror, it is difficult to read the visual information shown on the transparent substrate because of the reflection on the surface of the substrate.
Recently, screens have become bigger and wider with the development of display techniques. So, a clearer screen display is required, arousing interest in reflection prevention. If a reflection preventing function is added to a transparent substrate, a clearer screen can be secured by preventing reflection of the exterior light.
The conventional techniques to prevent the reflection on the surface of a transparent substrate include chemical or physical vapor deposition forming a transparent metal oxide layer on the surface of a transparent substrate. Those techniques can reduce the reflection of light over a wide range and at this time the reflection preventing effect is great. However, the deposition process is not suitable for mass-production because it has shown low productivity; therefore the process is less welcomed day by day.
As an alternative, a method for preparing a reflection preventing film using a coating solution containing silica or inorganic fine particles, a composition containing fluorine organic compound with low refractive index, or a copolymer containing fluorine organic compound and/or fluorine silane compound has been proposed.
In the case of preparing a film by using a coating solution containing silica or inorganic fine particles, the film is generally prepared by mixing porous particles with binder resin. At this time, the binder resin can be one of either silicon-based thermosetting resins with low refractive index or acryl-based photocurable resins that are able to reduce curing time. To prepare porous particles according to the conventional techniques, silica is hydrolyzed and condensed by using a structural regulator, similarly to a zeolite synthesis method, and the hydrolyzed and condensed silica is treated at a high temperature of at least 400˜500° C. to eliminate the structural regulator, resulting in the preparation of the porous particles. This method, however, has the disadvantages of requiring treatment at a high temperature of at least 400˜500° C., difficult redispersion for film with low reflective, inconveniently huge particle size, and thereby difficulty in preparing a transparent film with low refractive index. If the heat-treatment at a high temperature of at least 400˜500° C. is omitted, a structural regulator cannot be eliminated according to this method, so when the product is applied to a film with low refractive index which has to be prepared by low-heat treatment, for example a plastic substrate, the refractive index in fact increases.
To solve the above problems, an attempt has been made in which a film is formed by using silica particles containing a porous particle forming material such as a structural regulator and binder resin and then the porous particle forming material is eliminated by plasma treatment at low temperature or by dissolving it in an acid or solvent. However, plasma treatment of at least eight minutes is required to eliminate the porous particle forming material and the eliminated organic compounds become pollutants. The method of dipping a substrate in acid or solvent to eliminate the porous particle forming material also has the problem in that a wet process has to be added, making the method complicated and troublesome.
Another method for preparing porous particles is that particles are formed with aluminum and silicone and then aluminum is eliminated to form porous particles. However, this method has the problems of low yield resulting from the low silica concentration, overdose of solvent used for ultra-filtration, and prolonged manufacturing time.
In the case of using fluorine-based resin, the refractive index is reduced with the increase of the fluorine content. But, an over-dose of fluorine also causes a decrease in film adhesiveness, solubility in organic solvent, and coating properties, suggesting a limitation in lowering the refractive index.
According to the increasing demand of low dielectric materials in the semi-conductor industry, various methods have been tried to prepare an insulating film in the presence or absence of a porous particle forming material. However, all the methods are aimed at the promotion of the dielectric characteristics of a semiconductor and thus take the procedures of hardening of organic siloxane at high temperature and preparing a porous film by eliminating the organic compound at at least 350° C. That is, organic compound elimination at high temperature makes the method undesirable for the preparation of a low temperature oriented display film with low refractive index and reflection protection.
Therefore, a method is required for preparing photocurable or thermosetting film with low refractive index having excellent properties and which is characterized by easy manufacturing processes, excellent dispersibility, transparency, and low temperature processes.
To overcome the limitation of the conventional arts, the present inventors completed this invention by confirming that a film with extremely low refractive index can be prepared at a low temperature of up to 120° C. by adding a structural regulator to a silane compound for growing for further applications, and the film still maintains low reflection, dispersibility and transparency when it is mixed with silicone-based thermosetting resin or photocurable resin containing ethylenically unsaturated functional groups.